The majority of combustible fuel used in the world today is derived from crude oil. There are several limitations to using crude oil as a fuel source. Crude oil is in limited supply; it includes aromatic compounds that may be harmful and irritating, and it contains sulfur and nitrogen-containing compounds that can adversely affect the environment, for example, by producing acid rain.
Combustible liquid fuels can also be prepared from natural gas. This preparation involves converting the natural gas, which is mostly methane, to synthesis gas, or syngas, which is a mixture of carbon monoxide and hydrogen. An advantage of using products prepared from syngas is that they do not contain nitrogen and sulfur and generally do not contain aromatic compounds. Accordingly, they have minimal health and environmental impact.
Fischer-Tropsch chemistry is typically used to convert the syngas to a product stream that includes combustible fuel, among other products. These Fischer Tropsch products have very low levels of sulfur, nitrogen, aromatics and cycloparaffins. The Fischer Tropsch derived fuels are considered “green fuels” and are desirable as environmentally friendly.
Although environmentally friendly, these Fischer Tropsch products tend to oxidize relatively rapidly when exposed to air. The rapid oxidation may be due to a lack natural anti-oxidants, such as sulfur compounds. Further, some of the products produced by the Fischer Tropsch process may be waxy, and these products are frequently are shipped at elevated temperature. Shipping at elevated temperatures increases the tendency of Fischer Tropsch products to oxidize.
Various methods have been proposed to protect Fischer Tropsch products from oxidation during shipping and storage. For example, Berlowitz and Simon of Exxon Research and Engineering Company describe in World Patent Application Nos. WO 00/11116A1 and WO 00/111117A1 the blending of a Fischer Tropsch derived diesel fuel with high boiling sulfur containing streams, derived from gas field condensate or hydrotreated streams. Using the approach of Berlowitz and Simon to prevent oxidation adds high-boiling, sulfur-containing compounds to the Fischer Tropsch diesel fuel. Therefore, the products of Berlowitz and Simon contain sulfur, which prevents their use as low-sulfur, environmentally friendly fuels. Another undesirable feature of the products of Berlowitz and Simon is that a significant portion of the sulfur in those products is in the form of mercaptans (RSH). Mercaptans are well known to cause corrosion. Therefore, when shipping or storing products treated according to Berlowitz and Simon, corrosion of the large storage vessels can be a problem. Corrosion damage may lead to the need for eventual replacement of the large, expensive vessels used to ship and store hydrocarbonaceous products.
Various other well-known antioxidants may be used with Fischer Tropsch diesel fuels to prevent oxidation. These well-known antioxidants may include phenolic compounds and diphenylamine compounds. However, these antioxidants can be expensive when used on a large scale and must be transported to the remote site where the Fischer Tropsch diesel fuel is made.
There is a need for appropriate antioxidants for Fischer Tropsch derived products that do not impart polluting sulfur, corrosive mercaptans, or other undesirable components to the final product, and antioxidants that do not require shipping to the remote site where the Fischer Tropsch products are made. There is a need for efficient and economical methods of inhibiting oxidation of Fischer Tropsch derived products.